On the point of 3,000 MPH. One of the comments on the TechCrunch article maybe provides some insight on why they aren't targeting 17,500 MPH.
"even if they need to supplement with rocket propulsion (after exiting the rotational acceleration phase), the amount of rocket energy needed will be way less. could see a huge increase in payload mass fraction as a result. maybe closer to 20 or 30% instead of 1 or 2%"
IF they are flying beyond Mach 1, drag coefficient will actually drop as Mach number increases. [1] Gravity loss will play an interesting roll as well for a system launching with a high initial velocity. There will be a balance between aerodynamic drag and gravity drag losses. The lower the angle launched, the higher the aerodynamic drag but the low the gravity drag loss. [2] Typically launch vehicles actually need more than 9.5 km/s of delta-v even though the orbital velocity for low earth orbit is only 7.8 km/s. This is due to predominantly to account for the large drag forces the velocity experiences during the vertical portion of ascent at sub, trans, and lower super sonic speeds prior to exiting the atmosphere. Some allocation will need to be made for drag loss, but I suspect it is less than a traditional launch vehicle. I would also suspect there is a balance between launch speed, fuel fraction reduction, and difficulty of implementation. The sweet spot between those three things will be important to hone in on.
[1] https://en.wikipedia.org/wiki/Drag_(physics)#/media/File:Qua... [2] https://en.wikipedia.org/wiki/Gravity_drag